The FBI began an investigation yesterday of reports that an NBC television producer received a death threat on the opening day of the trial of a $150 million libel suit in Alexandria in which independent presidential candidate Lyndon H. LaRouche Jr. is suing the network.

Officials of NBC News, whose broadcasts describing LaRouche and his followers as "violence-prone" are the subject of LaRouche's suit, said a death threat was made Monday against the producer of one of the broadcasts.

The FBI was called in after NBC attorneys complained to U.S. District Judge James C. Cacheris, who is presiding at the trial.

In an interview with an FBI investigator and NBC attorneys last night in Northern Virginia, the producer, Pat Lynch, said a woman caller phoned NBC News in New York on Monday, the trial's opening day, and threatened her death if she testified during the trial.

Lynch said NBC Vice President Ed Planer phoned her at her Washington hotel room Monday night and told her the caller left a message for Lynch saying, ". . .if you testify you're dead."

Lynch said she and NBC officials called the phone number left by the caller. The person who answered her call, Lynch said, was a "frightened woman who said she had been getting funny calls." The woman refused to talk further with Lynch, Lynch said.

The trial stems from two NBC broadcasts aired earlier this year that cast the three-time presidential candidate as the anti-Semitic leader of "a political cult." The broadcasts, aired on the NBC Nightly News and the First Camera show, alleged that LaRouche and his followers engage in smear campaigns and routinely try to intimidate journalists who inquire about them.

LaRouche, who has not been present at the trial proceedings, will be listed on the November presidential election ballot as an independent in 19 states, including Virginia.

One of LaRouche's attorneys, Michael Dennis, called Lynch to testify yesterday afternoon for nearly three hours. During the questioning, Dennis pointed out that NBC did not interview LaRouche for the broadcast and used as sources people known to oppose LaRouche.

In response, Lynch testified that a decision was made not to interview LaRouche after LaRouche attempted to impose conditions on the interview.

Lynch, who has been an NBC producer off and on for eight years, defended the broadcasts, saying, "everything we heard from sources and interviews was that the LaRouche organization is violence-prone, anti-black, anti-Semitic."

Earlier in the day, Roy Innis, head of the national Congress of Racial Equality, testified that from his professional acquaintance with LaRouche, "I have seen nothing to substantiate those charges."

A taped interview in which Innis disputed the charges and called LaRouche "a breath of air on the political scene," was cut from the NBC broadcast, according to testimony.